This invention relates to a novel process for the production of dithiophosphoric acid polysulfide mixtures starting from dithiophosphoric acid disulfide.
Dithiophosphoric acid polysulfides are known, as is the use thereof as vulcanizing agents or vulcanizing accelerators for the vulcanization of rubber (c.f. DE 19 36 694, DE 22 49 090 and DE 44 31 727). Dithiophosphoric acid polysulfides may be produced from the corresponding dithiophosphoric acids or the alkali metal salts thereof and sulfur chlorides, such as disulfur dichloride or sulfur dichloride.
The disadvantages of reacting dithiophosphoric acids with sulfur chlorides include the elevated corrosiveness of the sulfur chlorides and the unpleasant odor thereof, which gives rise to handling problems and entails appropriate, complex plant and equipment. According to EP 0 383 102 A1, sulfur dichloride, moreover, very readily disproportionates, such that dithiophosphoric acid trisulfides are not directly obtainable using this process.
Additionally, the dithiophosphoric acid tetrasulfides obtained using the process described above have a tendency, due to the low stability thereof, to eliminate sulfur. In order to prevent this, it is necessary to stabilize the dithiophosphoric acid tetrachlorides against sulfur precipitation as is described, for example, in DE 44 31 727.
The object of the present invention is accordingly to provide a process for the production of dithiophosphoric acid polysulfide mixtures which avoids the use of sulfur chlorides and gives rise to sulfur-stable dithiophosphoric acid polysulfide mixtures.
The present invention accordingly provides a process for the production of dithiophosphoric acid polysulfide mixtures of the formula 
in which
R1 to R4 are identical or different and denote a linear or branched C1-C18 alkyl residue, C1-C18 alkenyl residue, C5-C28 cycloalkyl residue, C5-C28 cycloalkenyl residue as well as a C6-C28 aryl residues or C7-C28 aralkyl residue and
n denotes a number from 2.5 to 3.5,
which is characterized in that dithiophosphoric acid disulfides of the formula 
in which
R1 to R4 have the above-stated meaning,
are reacted with 0.5 to 1.5. mol of sulfur, optionally in the presence of a solvent, at temperatures of 100 to 140xc2x0 C.
The numeric values for n are determined statistically in accordance with the sulfur chain distribution in the molecule.
The residues R1 to R4 of the above-stated formulae may be substituted by suitable residues which are not disruptive to the subsequent use of the polysulfides, wherein alkyl residues and cycloalkyl residues may, in particular, be mentioned.
Preferred residues R1 to R4 are C6-C12 alkyl residues, in particular C8-C12 alkyl residues, in particular branched alkyl residues, such as 2-ethylhexyl.
The dithiophosphoric acid polysulfide mixtures produced according to the invention are preferably those in which n denotes numbers from 2.8 to 3.3.
The dithiophosphoric acid disulfides to be used in the process according to the invention are also known and are described, for example, in the Journal of Applied Polymer Science, volume 19, pp. 865-877 (1975). The disulfides are produced, for example, by oxidizing dithiophosphoric acid with hydrogen peroxide or HOCl or a mixture of potassium bromide and hydrogen peroxide.
In the process according to the present invention, the disulfides used are preferably reacted with 0.8 to 1.3 mol of elemental sulfur, wherein temperatures of 110 to 130xc2x0 C., in particular of 120 to 130xc2x0 C., are preferred.
If the reaction is to be performed in solution, the solvents used are in particular aliphatic solvents, for example naphtha, aromatic solvents, for example toluene, or halogenated aromatic solvents, such as chlorobenzene. The solvents may, of course, also be used as a mixture with each other.
The quantity of solvent may readily be determined by appropriate preliminary testing. Conventional quantities of solvent are from 0.1 to 50 wt %, relative to the weight of the disulfide used.
It is furthermore possible to add hydrogen peroxide to the reaction according to the invention in order to prevent any possible color changes. In this case, the hydrogen peroxide is used in quantities of 0.1% to 5 wt %, relative to the weight of the disulfide used.
Depending upon the reaction conditions employed, the reaction time ranges from approx. 10 minutes to approx. 6 hours.
Since the dithiophosphoric acid polysulfides produced according to the present invention are particularly sulfur-stable, i.e., they have no tendency to precipitate sulfur crystals, they are particularly suitable for use as sulfur donors for the vulcanization of natural and synthetic rubbers and for latex vulcanization of natural and synthetic rubber latex.